The present invention generally relates to centrifugal compressors and, more particularly, to centrifugal compressors with pipe diffusers.
Centrifugal gas compressors may be employed in numerous applications which may benefit from compact size of the compressor and a relatively low cost. It may be desirable to provide compact and low cost compressors in vehicles. For example, centrifugal compressors may be used as turbochargers in automotive applications or as air compressors in environmental control systems of aircraft.
A typical centrifugal compressor may employ a vaned wheel to draw gas from an inlet and accelerate the gas. As high-velocity gas emerges from an outlet of the wheel, the gas may enter a diffuser in which its velocity may be decreased and its static pressure may be increased. As the gas emerges from the diffuser, it may be in a compressed state.
Some centrifugal compressors may employ diffusers with vanes that form channels for reduction of gas velocity. These are known as vaned diffusers. Other centrifugal compressors may employ a solid diffuser in which tapered cylindrical openings act as velocity reduction channels. These tapered cylindrical openings are typically referred to as pipes. A diffuser which employs pipes is referred to as a pipe diffuser.
A typical pipe diffuser may be produced less expensively than a typical vaned diffuser. Consequently, it is desirable to build vehicular centrifugal compressors with pipe diffusers rather than vaned diffusers. The advantageously lower cost of a pipe diffuser has evolved largely as a result of fact that pipe diffusers may be fabricated from a single piece of metal with conventional machining techniques. While it is desirable to produce a centrifugal compressor at a low cost, it also important to assure that the centrifugal compressor may operate efficiently. It has been found that a smooth and controlled transition of gas from the wheel into the diffuser pipes is a key feature for providing high efficiency.
Referring now to FIG. 1, a cross-sectional view of a conventional centrifugal compressor 10 illustrates a relationship between a wheel 12 and a diffuser 14. The diffuser 14 may comprise an annular pipe diffuser section 14-2 and an integral shroud 14-4. In the compressor 10, a transition region 16 may exist between outer extremities of blades 12-2 of the wheel 12 and inlets 18-2 of pipes 18 of the diffuser 14. Gas 20 may enter the compressor 10 through an inlet 22. Rotation of the wheel 12 may accelerate the gas 20 along the blades 12-2 and drive the gas 20 toward the diffuser 14. The gas 20 may decelerate in the diffuser 14 as the gas 20 passes through the pipes 18. Passage of the gas through the pipes 18 may result in an increase in static pressure of the gas 20.
A design of the compressor 10 may be modeled mathematically to optimize its operational features. Such mathematical modeling may, for example, seek to minimize energy applied to the wheel 12 while maximizing static pressure produced in the pipes 18. In the context of the modeling, consideration may be given to a shape of the blades 12-2, a shape of the pipes 18 and also a shape of the transition region 16. After a mathematical model has been completed, resultant shapes for the blades 12-2 and the pipes 18 may be implemented with conventional fabrication techniques. For example, the wheel 12 and its blades 12-2 may be produced as a single metal casting. The pipes 18 of the diffuser 14 may be produced by conventional machining techniques such as drilling and honing.
Referring now to FIGS. 2 and 3, it may be seen that a mathematically defined shape of the transition region 16 shown in FIG. 1 may not be readily producible with conventional metal fabrication techniques. FIG. 2 illustrates a portion of the diffuser 14 during a fabrication step in which one of the pipes 18 may be produced as a hole in the annular pipe diffuser section 14-2 with a drill (not shown) and a honing tool 24. The honing tool 24 may produce a tapered hollow cylindrical shape for the pipe 18. In order to provide a complete shaping of any one of the pipes 18, the honing tool 24 may be required to project beyond an inner extremity or inlet 18-2 of the pipe 18 as illustrated in FIG. 2. When the honing tool 24 extends beyond the inlet 18-2, the tool 24 may produce tool marks or indentations 26 in the shroud 14-4 of the diffuser 14 as shown in FIG. 3. The honing tool 24 may also produce indentations 28 in a backplane 14-6 of the diffuser 14. The indentations 26 and 28 may result in a transition region 16′ having a shape that may be inconsistent with its mathematical model. Thus a configuration of the transition region 16′ of FIG. 3 may differ from a shape of transition region 16 of FIG. 1.
Referring now to FIG. 4, a collective effect of the indentations 28 may be seen. FIG. 4 may represent a cross-section of a portion of the diffuser 14 taken in a direction of the backplane 14-6. A dashed-line circle 30 may represent a desired radius at which the inlets 18-2 of the pipes 18 are to be located. The indentations 28 may produce an irregular surface on the backplane 14-6 of the transition region 16′.
As can be seen, there is a need to provide high efficiency in a centrifugal compressors with pipe diffusers even though the diffuser may be produced with low-cost fabrication techniques. In that regard there is a need to tailor compressor design to accommodate low cost fabrication techniques in order to optimize compressor efficiency and cost of fabrication.